1. Field of Invention
The present invention relates to the use of a microwave plasma processing device in the etching and film growth processes. More particularly, the invention is used in the process of manufacturing semiconductor devices. The plasma is generated by microwaves with electron cyclotron resonance ("ECR") excitation.
2. Description of Related Art
Several advantages are realized by devices in which plasma is generated by electron cyclotron resonance excitation. For example, a highly active plasma can be generated at low gas pressures, ion energies can be selected from a wide range of magnitudes, large ionic currents can be realized, and excellent directionality and homogeneity of ionic currents can be achieved.
FIG. 1 is a cross section of a conventional microwave plasma apparatus which is used for etching and which operates on the principle of electron cyclotron resonance excitation initiated by microwaves. FIG. 2 shows the constant strength flux density lines exemplary of the magnetic field of the conventional apparatus shown FIG. 1.
As shown in FIG. 1, plasma generating chamber 11 is comprised of cylindrical outer wall 11b which has coolant circuit 11a, upper wall 11c and lower wall 11d. Microwave feeding hole 12 is located at the center of upper wall 11c. Microwave feeding hole 12 is sealed with microwave feeding window 13 and is connected to a lower end of wave guide 14 at feeding window 13. The upper part of wave guide 14 is connected with a microwave generator (not shown). A generated microwave is introduced through wave guide 14 and feeding window 13 into plasma generating chamber 11. Gas feeding pipe 15 is connected with upper wall 11c. Electro-magnetic exciting coil 18 is coaxially placed around both plasma generating chamber 11 and the lower end of wave guide 14.
At lower wall 11d of plasma generating chamber 11, there is plasma extraction window 16. Located below plasma extraction window 16 is reaction chamber 17 which is connected with plasma generating chamber 11. Sample holder stage 19 for holding wafer S or some other sample, with an electro-static chuck or other conventional means for holding the sample in place, is positioned in reaction chamber 17 facing plasma extraction window 16. Pumping hole 20 at the lower wall of reaction chamber 17, is connected with an evacuation system (not shown).
The following describes the operation of an etching process with the above-mentioned conventional microwave plasma apparatus. First, plasma generating chamber 11 and reaction chamber 17 are evacuated. Following evacuation, gas is fed into plasma generating chamber 11 through gas feeding pipe 15. Electro-magnetic coil 18 creates a magnetic field within the apparatus. After formation of the magnetic field, a microwave is introduced into plasma generating chamber 11 through wave guide 14 and microwave feeding window 13. Plasma is then formed as a result of the resonance excitation initiated in the gas inside plasma generating chamber 11 which acts as a microwave resonant cavity. The plasma generated is transported and brought into contact with sample surface S in reaction chamber 17 by the force of the diverging magnetic field wherein the magnetic flux density of the magnetic field is decreasing in a direction toward reaction chamber 17 thereby ensuring that the surface of the sample S is etched by the plasma.
Constant strength magnetic flux density lines 21 of the apparatus shown in FIG. 1 are convex, as shown in FIG. 2, with respect to microwave feeding window 13 in plasma generating chamber 11 and concave with respect to sample S in reaction chamber 17. At the midpoint of a line drawn from feeding window 13 to sample S, constant strength magnetic flux density lines 21 approach complete flatness. The region along axis A where constant strength magnetic flux density lines 21 are flat is small and the gradient of the magnetic field is steep. With the resulting magnetic field configuration, the ratio of the microwave power absorption efficiency to the amount of plasma is low. Absorption is large at the center of the device causing the plasma density distribution and the stability to be poor.
The conventional apparatus shown in FIGS. 1 and 2 provides an 875 G region which is thin in axial direction A since the device generates a non-flat ECR condition. Constant strength magnetic flux density line 22 represents the 875 G region. In this case, a very small area of the ECR layer is flat with respect to a plane perpendicular to axial direction A. As mentioned above, this provides high microwave absorption only at the center of plasma generating chamber 11 with the result that plasma density distribution and stability is poor. Hereafter, such an arrangement is referred to as the "non-flat ECR condition."
U.S. Pat. No. 4,911,814 ("the '814 patent") discloses a thin film forming apparatus and ion source utilizing sputtering with microwave plasma. In particular, the '814 patent discloses a sputtering type ion source utilizing ECR plasma. Several embodiments of the apparatus disclosed in the '814 patent include two vertically spaced-apart electromagnets above and below an entry hole in a side of a plasma chamber through which microwave energy is introduced. The electromagnets are arranged such that a magnetic field sufficient to cause ECR is produced and the magnetic flux passes between two targets in the plasma generating chamber. The currents passing through the electromagnets can be 20A each or the current passing through the electromagnet closest to the reaction chamber can be 20A whereas the current passing through the other electromagnet can be 10A. The '814 patent does not disclose the shape of the ECR layer nor is there any suggestion of supplying a lower amount of current to the electromagnet located closest to the reaction chamber.
Other ECR type plasma treating apparatuses are disclosed in U.S. Pat. Nos. 4,401,054; 4,438,368; 4,806,829; 4,859,908; 4,947,085; 4,973,883; and 4,990,229.
There is a need in the art for a microwave plasma generating device which provides a strong, uniform and stable plasma. Applicants have developed an apparatus which produces an improved magnetic field distribution resulting in enhanced microwave power efficiency in the plasma and improved uniformity with respect to plasma density, absorption distribution uniformity.